At present, the high-pressure mercury vapor discharge lamp is very useful as a highly reliable light source which has high efficiency and long lifetime characteristics. However, the color-rendering properties of the high-pressure mercury lamp are as poor as approximately 23 in the general color rendering index Ra. (FIG. 1 shows the spectral distribution of this high-pressure mercury vapor discharge lamp). Its usage has therefore been limited to exterior lighting and similar purposes. In order to enhance its color-rendering properties while maintaining its high efficiency and long life characteristics, the so-called fluorescent high-pressure mercury vapor discharge lamp was introduced and put into practical use. In this type of lamp the inside wall of its outer jacket is coated with a layer of a red emitting phosphor, comprising, for example, yttrium vanadate activated by europium. The color-rendering properties of the lamp have been gradually enhanced by improvement of the phosphor and related applied technology, but phosphor improvement alone has achieved only a rather poor general color rendering index value of approximately 53. (FIG. 2 shows the general spectral distribution of this type of fluorescent, high-pressure mercury vapor discharge lamp.) Another attempt to improve the color-rendering properties of the high-pressure mercury discharge lamp was made in the past, as an alternative to the above-mentioned use of phosphors in the lamp. In the alternative technique some additional metals were enclosed in the arc tube besides mercury such as cadmium and zinc so that the emission spectrum resulting from the discharge of the vapor of such metals would add to that from the mercury vapor discharge to improve the overall color-rendering properties of the lamp. However, it has been found, as discussed in the book edited by Elenbaas (W. Elenbaas, editor, "High Pressure Mercury Vapor Lamps and Their Applications", 1965, p. 294 ) that the problem with such lamps is, if a satisfactory emission spectrum is to be obtained by vapor discharge of cadmium or zinc, the vapor pressure level of such a metal must be raised, and the only way to do this is to set the designed temperature at the wall of the arc tube substantially high, and the wall loading of the arc tube higher than before. Since the arc tube must be made of silica, the arc tube at high temperature is susceptible to erosion by cadmium or zinc vapor, until the tube eventually cracks or is otherwise damaged which quite substantially shortens the life of the discharge lamp. In view of these facts, the present inventors for a time focussed attention on the color rendering property alone, and experimented by adding cadmium or zinc to the arc tube in attempts to fabricate a high-pressure vapor discharge lamp, which failed to show satisfactory color-rendering property with a general color rendering index of approximately 60. (FIG. 3 shows the spectral distribution of the high-pressure mercury vapor discharge lamp enclosing cadmium.) Moreover, because the load to be borne by its arc tube rose to a level of more than 50 percent above the conventional high-pressure mercury vapor discharge lamp, its arc tube broke down after approximately 1,000 hours had lapsed.
Another method to improve color-rendering property is to utilize the so-called metal halide discharge lamp, in whose arc tube the halides of various metals are additionally enclosed so as to obtain high vapor pressure without increasing tube wall temperature substantially. While good color rendition is now secured by the metal halide lamp, its weaknesses is its incapability of being lit by ordinary type ballasts for high pressure mercury vapor discharge lamps, because the starting voltage is higher than that of the conventional mercury lamp, because of the fact that it cannot use electrodes with an alkaline earth metal oxide electron emitter whose feature is good electron emissivity and that impure gases are likely to be admitted into the arc tube when the halide is enclosed in the tube. As a result, the metal halide lamp requires for its starting a specially designed ballast, which is large, heavy and expensive, all of which are defects which hamper its wide use.